A digital imager is a system capable of detecting a source image and of converting said image into a matrix of numbers that is also what is called the output image. This output image is then displayed on a screen to an operator who will make decisions or perform measurements on the basis thereof, or who will simply appreciate the visual quality of the image in the case of an artistic or familial use.
In the case of x-ray imagers, a clinical image is produced by interposing a patient between an x-ray source and the imager. An image of the transmittance of the patient to x-rays is obtained. The operator is a radiologist or a radiographer.
A digital imager generally implements one or more of the following phases to generate the output image: transduction, sampling, digitization, correction and/or presentation.
The transduction is the conversion of the source image into what is called a capturable image, i.e. an image capturable by electronics: for example an image of visible photons, or an image of electrical charges. Certain imagers have no need for transduction because they can capture the source image directly (for example capture of a visible image by a CCD sensor (CCD standing for charge-coupled device)).
In the case of certain flat x-ray images, the x-ray photons from which the source image is formed are converted into visible light by a scintillator.
This electronic image is then sampled in the form of a matrix. This is what is called sampling. This sampling matrix may actually exist (case of a matrix array of TFT pixels) or may be created virtually during the process of capturing the information. Mention may be made of the case of a vacuum tube that scans a surface with an electron read beam sampled at regular time intervals.
In the case of certain flat x-ray imagers, the sampling is carried out by a matrix array of photosensitive pixels.
During the digitization, the analogue information is converted into digital information, which information is delivered by read-out electronics, and arranged into an image in accordance with the sampling matrix. This image is called the raw image. It contains dimensionless values but they are by convention considered to be expressed in LSB (acronym for least significant bit).
The raw image is not directly proportional to the source image. It must undergo a correction. It is therefore processed to make it as faithful a reproduction of the source image as possible, i.e. transformed by calculation to be directly proportional to the source image. This new image is called the clean image or even the pre-processed image. The processing operations carried out are referred to as offset correction, gain correction and defect and artefact correction. These processing operations are detailed below.
The clean image is modified to create an image better adapted to the needs of the operator, which is called the post-processed image. This is what is called the presentation step.
In the case of flat x-ray imagers for radiography, the presentation consists in sharpening outlines, compressing dynamic range (logarithmic scale), and level inversion (to make the image resemble a conventional radiograph as it would appear on an x-ray film). Other modalities (fluoroscopy for example) apply different processing operations.
The raw image will be called A. If each pixel of the matrix is being referred to, Aij is spoken of, the suffix ij corresponding to the pixel of row coordinate i and of column coordinate j. The source image will be called S, and by the same convention, Sij is spoken of. Generally, these two images are related pixel to pixel by the following formula:Aij=Sij×Gij+OFij  (1)
This formula is true for any digital imager, at least in a certain range of values of Sij, and for most of the pixels thereof. This range is called the range of linearity of the imager. G and OF are respectively called the gain and offset of the imager.
The first step of the correctional processing operations is the correction of offset. The result of correcting the offset in A is an image called A_Oc. The calculation carried out is:A_Ocij=Aij−OF_REFij  (2)
The image OF_REF is called the offset reference. This image must be measured for each detector by the user of the digital imagery system, with what is called an offset calibration procedure. The offset reference may initially be measured by acquiring dark images.
In the field of x-ray imagers, a dark image is produced without exposure to x-rays. To determine the offset reference, a dark image, or a plurality of dark images that are averaged in order to minimize the residual noise in the reference offset image, is or are produced. This procedure is carried out either some time (i.e. a few minutes or a few hours) before the x-ray imager is used in clinical service, or just before or just after the acquisition of a clinical image. During the implementation of this procedure, the detector is not available for acquiring clinical images.
By inserting equation (1) into equation (2), the following is obtained:A_Ocij=Sij×Gij+(OFij−OF_REFij)  (3)
If the offset calibration has been carried out correctly, then:(OFij−OF_REFij)˜0,  (4)
where “˜” means “approximately equal to”, and therefore:A_Ocij=Sij×Gij  (5)
At this stage, the image A_Oc corrected for offset is obtained.
The following step is the correction of gain. The result of correction of the gain of A_Oc is an image called A_OcGc. The calculation carried out is:A_OcGcij=A_Ocij/G_REFij  (6)
The image G_REF is called the gain reference. This image must be measured for each detecter by the user of the digital imagery system, using what is called a gain calibration procedure. To determine the gain image, a plurality of white images corrected for offset are produced and the gain image is calculated by taking the average of these images. The aim of this averaging is to minimize the residual noise in the gain image. In the field of x-ray imagers, white images are produced by exposing the imager to x-rays without interposing either a patient or an object between the imager and the radiation source. This procedure is carried out some time (minutes, hours, days or years) before the x-ray imager is used in clinical service and requires the detector to be taken out of service for as long as the calibration takes.
By inserting equation (5) into equation (6), the following is obtained:A_OcGcij=Sij×Gij/G_REFij  (7)
If the gain calibration has been carried out correctly, then:Gij/G_REFij˜k  (8)
where k is a constant that is identical for all the pixels of the image; and therefore:A_OcGcij=Sij×k  (9)
The image A_OcGc corrected for offset and gain is therefore a faithful reproduction of the image S, i.e. is equal to S, to within a factor k.
Certain pixels of the imager do not satisfy formula (1) and after step (9) have a value that is not a faithful reproduction of S. If the deviation is excessively large, these pixels are considered to be defective, and it is chosen to replace their value with another value. This calculation is what is called defect correction. For example, the defect correction may consist in replacing the value of the defective pixel with the average of the 4 or 8 nearest non-defective neighbors. The defect correction is in general carried out after the offset and gain correction, on the image A_OcGc, and the resulting image is called A_OcGcDc.
To carry out the defect correction, it is necessary to know the list of defective pixels. The procedure, the aim of which is to determine the list of defective pixels, is called the defect calibration. It is for example possible to search, in the images OF_REF or G_REF, for pixels having an atypical value, and to decide that these pixels are defective. In the case of x-ray imagers, this procedure is carried out some time (minutes, hours, days or years) before the x-ray imager is used in clinical service and requires the detector to be taken out of service for as long as the calibration takes.
The list of defects may be stored in an image called DM_REF, the pixels of which are assigned a value of 0 when they are defective and a value of 1 otherwise.
Certain pixels of the imager do not satisfy formula (1) and after step (9) have a value that is not a faithful reproduction of S. However, the deviation is sufficiently small for them not to be considered to be defective. These pixels or groups of pixels however generate an infidelity that may be seen in the image: artefacts are then spoken of. Artefacts may take various forms and be of various amplitudes, for example an artefact may be a row that is slightly whiter than the others. When these artefacts are known and have a sufficiently predictable and specific behavior, it is possible to remove them from the image by calculation, without degrading the fidelity to the source image. This operation is called artefact correction. In contrast to defect correction, artefact correction may be carried out at any stage of the correction, i.e. on image A or A_Oc or A_OcGc or A_OcGcDc. The most suitable step is chosen, depending on the artefact generation process. For example, an additive artefact is corrected in image A_Oc and a multiplicative artefact is corrected in image A_OcGc or A_OcGcDc.
To correct artefacts it is necessary to determine their number, position and amplitude. This is done in a procedure called artefact calibration. The calibration of an artefact often consists in placing the imager under specific conditions that make the artefact and no other effects appear, in order to obtain a reference image A_REFij for the artefact.
In the field of x-ray images, an exemplary artefact is pixelization with temperature, for which patent FR0707563 has been granted. This artefact is a local drift in gain that depends on temperature and that cannot therefore be captured in the image G_REFij, which is produced at a single temperature. The patent describes a method for measuring two gain references at two different temperatures, thereby allowing the calculation of a gain correction to be applied to the affected zones only, on the basis of a temperature delivered by a temperature sensor integrated into the imager. This gain correction is different for each pixel, thereby causing it to take the form of a image A_REFij of sensitivity to temperature. This procedure is carried out some time (minutes, hours, days or years) before the x-ray imager is used in clinical service and requires the detector to be taken out of service for as long as the calibration takes.
As was seen above, the corrections require calibrations. These calibrations consist in producing reference images, OF_REF, G_REF, DM_REF or A_REF, that are generically called C_REF below. The calibration procedures pose a number of problems.
They take the imager out of service for as long as they take. Specifically, calibration procedures in general require images to be produced under very controlled conditions, without disruption, this being incompatible with normal use of the imager.
They may require the intervention of an operator who is specialized in the implementation of these procedures, for example because a procedure is complex or requires specific tools (calibration test patterns for example). In the case of an x-ray imager, the conventional procedure for calibrating gain requires images to be taken with x-rays, and, in this case, standards in force require a human operator to be present.
The calibration is sometimes not stable over time and may require relatively frequent recalibrations. The cause of this instability may be reversible or irreversible changes in the constituent hardware of the detector, or variations in the environment outside the detector (temperature, humidity, pressure). In the case of x-ray imagers, offset is recalibrated as often as possible, sometimes even between each clinical image. Defects may require an equally frequent or less frequent (once per year) recalibration. Gain may require recalibration several times per day or once per year. As regards artefacts, it is very different depending on the type of artefact: the frequency may vary from once per image to once per year.
These problems cause the user to incur additional costs: downtime cost and the cost of human intervention. This may lead in certain cases to the detector not being re-calibrated as often as necessary by the user, the final quality of the produced images therefore being degraded.
It would therefore be very useful to have at one's disposal a calibrating procedure not having the aforementioned drawbacks, i.e. one having the following features:
the feature of not taking the detector out of service, i.e. the feature of also being able to work with images disrupted by normal imager use;
the feature of not requiring human intervention. In the case of an x-ray imager, this in particular means carrying out the gain calibration with clinical images;
the feature of allowing very frequent recalibration: by virtue of the 2 preceding points, the cost of calibrating operations is decreased and therefore it is possible to carry out recalibrations as frequently as necessary, thereby guaranteeing the detector delivers a maximum image quality throughout its lifetime.